Nereid (astronomy), large moon of the planet Neptune. Nereid orbits Neptune at an average distance of about 5.51 million km (about 3.33 million mi), completing an orbit about once every 360 Earth days. Nereid is about 12 times more distant from Neptune than the largest moon, Triton. Of all major moons in the solar system, Nereid has the most elliptical orbit around its planet. When Nereid is at its closest to Neptune, its distance from the planet is only about 1.35 million km (about 812,000 mi). At the farthest point in its orbit around Neptune, Nereid is about 9.624 million km (about 5.77 million mi) from the planet. Nereid’s orbit is tilted 28° relative to Neptune’s equator. Nereid orbits counterclockwise, in the same direction that Neptune turns on its axis. By contrast, Triton has a clockwise orbit.

Nereid is thought to be irregular in shape, based on studies of variations in its reflected brightness (albedo) as measured using photometry from telescopes on Earth. Observed over long periods, the moon shows the largest variations in brightness of any body in the solar system. Estimates suggest its average diameter is about 340 km (about 204 mi), or about one-tenth as wide as Earth’s moon. Nereid’s period of rotation is not known, but one theory is that the direction that its axis points precesses (shifts over time) because the pull of Neptune’s gravity changes at different points in the moon’s highly elliptical orbit. Nereid’s internal structure, density, and composition are unknown and little is known of the moon’s surface. The United States Voyager 2 probe, the only spacecraft to visit Neptune, came no nearer to the moon than about 4.7 million km (about 2.9 million miles). Only a few large craters are visible in the pictures taken by Voyager 2.

Some planetary scientists believe that Nereid has such an odd orbit because it was originally a member of the Kuiper Belt (a number of icy bodies orbiting the sun beyond Neptune) or a comet. Others theorize that Nereid’s orbit was perturbed when Neptune captured Triton. Nereid may always have been a moon of Neptune, but could have been thrown into its strange orbit by Triton’s gravity when the two moons nearly collided following Triton’s capture.

Nereid was discovered in 1949 by Dutch-American astronomer Gerard Kuiper. The moon was named for a group of fifty sea nymphs who were daughters of the Greek god Nereus and his wife, Doris. Individual features on Nereid are named for the individual Nereids.